System in package process flow

ABSTRACT

A method comprises connecting a substrate having a plurality of integrated circuit (IC) dies to a package substrate, so that the package substrate extends beyond at least two edges of the substrate, leaving first and second edge portions of the package substrate having exposed contacts. The first and second edge portions meet at a first corner of the package substrate. At least a first upper die package is placed over the substrate, so that first and second edge portions of the first upper die package extend beyond the at least two edges of the substrate. Pads on the first and second edge portions of the first upper die package are connected to the contacts of the first and second edge portions of the package substrate.

This application is a continuation-in-part of U.S. patent application Ser. No. 13/290,208, filed Nov. 7, 2011, which is incorporated by reference herein in its entirety.

FIELD

This disclosure relates to semiconductor integrated circuits (ICs) and methods for packaging the ICs.

BACKGROUND

As the semiconductor industry continues to reduce package dimensions, foundries are looking to increase vertical density in addition to horizontal circuit density. 3D packaging saves space by stacking separate chips in a single package. This packaging, known as System in Package (SiP), uses off-chip signaling (e.g., by way of conductive paths in the package substrate) for communication between chips in the package The chips are not integrated into a single circuit. Rather, the chips within in the package communicate as though they are mounted in separate packages on a normal circuit board. For example, both the upper and lower chips may be wire bonded to the package substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of a package substrate.

FIG. 1B is a top plan view of a substrate strip including 48 of the substrate of FIG. 1A.

FIG. 1C shows a side cross sectional view of one of the die mounting regions of FIG. 1B, with a bottom die being mounted thereon.

FIG. 1D is a plan view of the substrate of FIG. 1B, after the bottom dies are mounted thereon.

FIG. 1E is a bottom plan view of a wafer level chip scale package.

FIGS. 2A to 2F are cross sectional views showing various stages of a method of fabricating a System in Package according to one embodiment.

FIGS. 3A and 3B are top plan views of the package shown in FIG. 2F.

FIGS. 4A-4D are cross sectional views showing various stages of a method of fabricating a System in Package according to a second embodiment.

FIGS. 5A-5D are cross sectional views showing various stages of a method of fabricating a System in Package according to a third embodiment.

FIGS. 6A-6F are cross sectional views showing various stages of a method of fabricating a System in Package according to a fourth embodiment.

FIG. 7 is a flow chart of an SiP packaging method.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The drawings are not drawn to scale.

FIGS. 2F and 3A show a first embodiment of an integrated circuit (IC) package 200. FIGS. 1A, 1C (800 bottom die) and 1E show the major components included in the package 200. (or say, 100, 800 and 180 show the major components included in the package 200.)

FIG. 1A shows a package substrate 100. The package substrate 100 comprises a package substrate dielectric layer 102 which may be made from a dielectric such as FR-4 (Woven glass and epoxy), or other prepreg materials such as FR-2 (Phenolic cotton paper), FR-3 (Cotton paper and epoxy), FR-5 (Woven glass and epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and epoxy), CEM-5 (Woven glass and polyester). In other embodiments, the package substrate dielectric layer 102 may be made from polytetrafluoroethylene.

Package substrate 100 has a plurality of conductive patterns and vias, collectively labeled 104, for distributing signals and power between the ICs above and the solder balls 188 below. On the top surface of package substrate 100, a plurality of solder balls or bumps 112 are provided for connections between the package substrate 100 and a substrate 150 (shown in FIG. 1B). In alternative embodiments, instead of solder 112, the substrate 100 may have bare pads (e.g., copper pads), for interconnection with a die flip-chip mounted above the substrate). A plurality of pads 110 are provided for connections between the package substrate 100 and an upper die package 180 (shown in FIG. 1E).

In FIG. 1B, a substrate design assembly 150 provides the main package substrate. The die mounting region 157 includes a portion of the substrate 157 a, with solder bumps 157 b on contacts for connecting a bottom die 800 (shown in FIG. 1C). One substrate 150 may have many die bonding areas 157, grouped in blocks 159. In some embodiments, the substrate 150 has one to four blocks 159. Every block 159 may have from 9 to 20 bonding areas for bottom dies 800.

FIG. 1C is a side cross section view of one of the die mounting regions 157 of FIG. 1B, and is representative of all of the regions. A respective die 800 is connected to substrate 150 in each respective one of the die mounting regions 157. The substrate 150 includes additional bumps 157 c for connecting top dies 180 above the bottom die 800. The bumps 157 c are arranged around the perimeter of the die mounting region 157, so as to be exposed after die 800 is mounted on the substrate. Although not shown in FIG. 1C, a TIM 160 (as shown in FIG. 2B) is provided over the bottom die 800.

Next, as shown in FIG. 1D, a plurality of top dies 180 are mounted over the bottom dies 800. The top dies 180 may be wafer level chip scale packages (WLCSP, as described above) to join the bottom dies and substrate together, and the assembly 850 is encapsulated with an encapsulant (e.g., molding compound) to complete the package-on-package, (PoP) assembly 850, shown in FIG. 1D. In some embodiments, one top die 180 is mounted over each bottom die 800. In other embodiments, two or four top dies 180 are mounted over the bottom dies 800, in configurations as shown in FIGS. 3A and 3B. Thus, substrate 150 may serve as the package substrate of the PoP assembly 850, and a separate package substrate (e.g., 102 in FIG. 2A) is not included.

Thus, FIG. 1D shows an integrated circuit (IC) package 850 comprising a substrate 150 having a plurality of bottom dies 800 connected to the substrate 150, so that first and second connecting portions 159 of the substrate 150 extend beyond at least two edges of each bottom die 800, the first and second connecting portions 159 meeting at a first corner of each bottom die 800. at least a first upper die package 180 is positioned over at least one of the bottom dies 800, so that first and second edge portions of the first upper die package 180 extend beyond the at least two edges of at least one of the bottom dies 800. The first upper die package 180 has pads on the first and second edge portions thereof (as described above). The pads are connected to the contacts of the first and second connecting portions 159 of the substrate 150 by solder 157 c.

Although the configuration of FIG. 3B shows four top dies 180 per substrate 150, the configuration of FIG. 1D may have many more top dies 180. For example, FIG. 1D shows 48 top dies 180 above one substrate 150. The configuration of FIG. 1D also differs from that shown in FIG. 2F in that, in FIG. 1D, the top dies 180 are mounted above the front face of the substrate 150 (i.e., the face having the bumps for connecting dies to the substrate 150). In the configuration of FIG. 2F, on the other hand, the top dies 180 are mounted above the back face of the substrate 150.

FIG. 1D shows a substrate 150 having a plurality of IC dies 800 connected to a substrate package substrate 156. One substrate 150 may have many bond areas for bonding respective IC dies 800 onto the substrate 150. The substrate 150 may be, for example, any of the packages described in U.S. Patent Application Publication No. US 2008/0230883 A1, which is incorporated by reference herein in its entirety. The substrate 150 includes an in-line, such as a copper lead frame array (not shown). The in-line includes individual units 800, having in-line rails 156 such as guide rails or side rails. The in-line rails 156 guide the in-line 150 in feeding the in-line 150 into processing equipment or handling equipment.

The package substrate 100 and substrate 150 are sized, so that first and second edge portions 101 a, 101 b of the package substrate 100 extend beyond at least two edges 151 a, 151 b (shown in FIG. 3A) of the substrate 150, where the first and second edge portions 101 a, 101 b meet at a first corner of the package substrate 100. In FIG. 3A, edges 151 a-151 d are shown in phantom, indicating that the edges 151 a-151 d are hidden beneath the exposed surface of the package.

FIG. 1E shows an upper die package 180, which may be a wafer level chip scale package. At least one upper die package 180 is provided. Various embodiments may include one, two, three or four upper die packages 180 depending on the number of functions to be performed, or amount of storage to be provided, by the upper die package configuration. For example, the at least one upper die package can include one or more of the group consisting of a general purpose processor (e.g., a system on chip or SoC), a graphics processor, an audio/video processor, a digital radio receiver, DRAM, SRAM, or flash memory, a communications processor (which may include a WiFi (802.11), Bluetooth, or broadband interface), a global positioning satellite (GPS) receiver, or other communications interface; a second processor and a power management unit.

The first upper die package 180 has connectors 183 on the first and second abutting edge portions 182 thereof. The first and second edge portions 182 form an L shaped region along two side edges 181 a, 181 b of the first upper die package 180. In some embodiments, the first upper die package 180 has solder balls 186 on the connectors 183. In other embodiments, the first upper die package has connectors 183 without solder balls. A second region 184 of the first upper die package 180 has no pads. The second region 184 is adapted to be used as a mounting interface surface.

At least the first upper die package 180 is to be positioned over the substrate 150, so that first and second edge portions 181 a, 181 b of the first upper die package 180 extend beyond the at least two edges 151 a, 151 b of the substrate 150. A thermal interface material 160 is positioned between the substrate 150 and the second region 184 of the first upper die package 180. The thermal interface material 160 (optional) may be a soft polymer, such as a polyimide, or air only.

In the final package configuration of FIG. 2F, the connectors 183 are connected to the contacts 110 of the first and second edge portions 101 a, 101 b of the package substrate 100 by solder 186.

FIG. 3A shows an optional second upper die package 180. The package substrate 100 extends beyond third and fourth edges 151 c, 151 d of the substrate 150 opposite the first and second edges 151 a, 151 b of the substrate 100. The third and fourth edge portions 181 c, 181 d of the second upper die package 180 extend beyond third and fourth edges of the substrate 150. The third and fourth edge portions 151 c, 151 d of the second upper die package 150 have pads connected to the contacts of the third and fourth edge portions 101 c, 101 d of the package substrate 100.

In FIG. 3A, the two upper die packages 180 are shown as being of the same size. This is optional. In some embodiments, the first and second upper die packages 180 are configured identically to each other, for example to provide two DRAM packages.

In other embodiments (such as shown in FIG. 3B), the upper die packages may be of different sizes, and may contain different types of ICs for performing distinct functions. In the embodiment 300 of FIG. 3B, the first, second, third and fourth upper die packages 180 are wafer level chip scale packages.

As shown in FIG. 3B, a third upper die package 180 may optionally be provided over the bottom die 800, having fifth and sixth edge portions 181 e, 181 f of the third upper die package extending beyond the second and third edges 151 b, 151 c of the substrate, the fifth and sixth edge portions 181 e, 181 f of the third upper die package having connectors 183 connected to the contacts 110 of the second and third edge portions 101 b, 101 c of the package substrate 100. A fourth upper die package 180 over the bottom die 800 has seventh and eighth edge portions 181 g, 181 h of the fourth upper die package extending beyond the first and fourth edges 151 a, 151 d of the bottom die 800. The seventh and eighth edge portions 181 g, 181 h of the fourth upper die package 180 have connectors 183 connected to the contacts 110 of the first and fourth edge portions 101 a, 101 d of the package substrate 100.

FIGS. 2A to 2F show a first example of a method for fabricating the package 200. FIG. 7 is a flow chart of the method

At step 702, a bottom die 800 having a plurality of integrated circuit (IC) dies 800 is connected to a package substrate 100, so that the package substrate 100 extends beyond at least two edges 151 a, 151 b of the substrate 150, leaving first and second edge portions 101 a, 101 b of the package substrate having contacts thereon exposed, where the first and second edge portions 101 a, 101 b meet at a first corner of the package substrate. In the example of FIG. 2A, third and fourth edge portions 101 c, 101 d of the package substrate 100 are also exposed. The bottom die 800 is mounted to the package substrate 100. The bottom die 800 is positioned so that the pads of the bottom die 800 confront the solder balls 112 of the package substrate 100. The assembly is heated to reflow the solder balls 112, forming the electrical connections to the conductive patterns and vias 104 of the package substrate 100.

At step 704, a first underfill material 120 is flowed into the space between the package substrate and the substrate 150. The resulting configuration is as shown in FIG. 2A.

At step 706, as shown in FIG. 2B, a thermal interface material (TIM) 160 is placed over the back surface of the substrate 150. The TIM 160 may be printed or stenciled onto the dies 150 or applied as a TIM film, so that the substrate is covered, but the contacts 110 of edge portions 101 a-101 d of the package substrate remain exposed.

At step 708, as shown in FIG. 2C, a solder paste 170 is deposited on the contacts 110 of edge portions 101 a-101 d of the package substrate 100. The solder paste 170 may be defined by stencil or deposited by a solder jet.

At step 710, as shown in FIG. 2D, at least a first upper die package 180 is placed over the bottom die 800, so that first and second edge portions 181 a, 181 b of the first upper die package 180 extend beyond the at least two edges 151 a, 151 b of the bottom die 800. The portion 184 of the first upper die 180 having no connectors 183 at least partially overlies the bottom die 800, and directly contacts the TIM 160. Thus, the first upper die partly overlies and extends beyond the corner of the bottom die 800.

In the example of FIG. 2D, at least two upper die packages 180 are placed over the substrate 150. The package substrate 100 extends beyond third and fourth edges 151 c, 151 d of the bottom die 800 opposite the at least two edges 151 a, 151 b of the substrate, leaving third and fourth edge portions 101 c, 101 d of the package substrate 100 having contacts 110 thereon exposed. A second upper die package 180 is placed over the substrate 150, so that third and fourth edge portions 181 c, 181 d of the second upper die package 180 extend beyond the third and fourth edges 151 c, 151 d of the substrate 150. The bare surface portions 184 of the upper die packages 180 directly confront the TIM material 160 and pressure is applied to eliminate any air gaps, thus ensuring good thermal conductance between the bottom die 800 and the upper die packages 180.

At step 712, the connectors 183 (with balls 186) on the first and second edge portions 181 a, 181 b of the first upper die package 180 are connected to the contacts 110 of the first and second edge portions 101 a, 101 b of the package substrate 100. The connectors 183 on the third and fourth edge portions 181 c, 181 d of the second upper die package 180 are connected to the pads 110 of the third and fourth edge portions 101 c, 101 d of the package substrate 100. In the method of FIGS. 2A-2F, the upper die packages 180 are provided with solder balls 186, copper posts, or conductive spacers on the connectors 183. The electrical and mechanical connections are made by heating the assembly to reflow the solder balls 186.

At step 714, the assembly is cleaned, for example by flushing with deionized water, to remove any remaining solder paste and/or flux.

At step 716, as shown in FIG. 2E, a molded underfill material 190 is applied around the solder 186 and bottom die 800, and above the first and second upper die packages 180, filling the space between the bottom die 800 and the package substrate 100.

At step 718, solder bumps 188 are applied to the package 100.

In some embodiments, the package is completed at the end of step 2E

In other embodiments, the package 200 is thinned by a planarization process (step 720), to remove the portion of the molding compound 190 above the back faces of the upper die packages 180, as shown in FIG. 2F. Optionally, the package 200 may be thinned further, to remove more material from the back face of the upper die packages 180 and thin the die packages 180. Removal of the molding compound material 190 above the upper die packages and optionally thinning the upper die packages improves the thermal performance of the IC package 200.

Although the method of FIGS. 2A-2F is described above with reference to an embodiment having two upper die packages 180 (as shown in FIG. 3A), FIGS. 2A-2F also show the process used for an assembly having three or four upper die packages (as shown in FIG. 3B) or one die package. The method is the same, except that in step 710, three or four upper dies 180 are placed in FIG. 2D, instead of two upper dies, and the solder for the third and fourth upper dies is reflowed at step 712.

Thus, step 710 includes placing a third upper die package 180 over the bottom die 800, so that fifth and sixth edge portions 181 e, 181 f of the third upper die package extend beyond the second and third edges 151 b, 151 c of the substrate; and placing a fourth upper die package over the substrate, so that the seventh and eighth edge portions 181 g, 181 h of the fourth upper die package extend beyond the first and fourth edges 151 a, 151 d of the substrate. In this embodiment, step 712 further includes connecting pads on the fifth and sixth edge portions 181 e, 181 f of the third upper die package to the contacts 110 of the second and third edge portions 101 b, 101 c of the package substrate; and connecting pads on the seventh and eighth edge portions 181 g, 181 h of the fourth upper die package to the contacts 110 of the first and fourth edge portions 101 a, 101 b of the package substrate 100.

FIG. 7 shows one sequence for performing the packaging steps. The sequence of performing the various steps shown in FIG. 7 may be altered. Three alternative sequences are described below.

FIGS. 4A to 4C show a variation of the fabrication method.

In FIG. 4A, the assembly shown in FIG. 2B is provided. The steps performed to provide this assembly are described above, and for brevity the description is not repeated.

In FIG. 4B, the solder balls 186 are placed on the contacts 110 of edge portions 101 a-101 d of substrate 100, and reflowed to join the solder balls and contacts 110.

In FIG. 4C, the solder paste 170′ is applied over the solder balls (as opposed to solder paste 170 under the solder balls in FIG. 2C). The solder paste 170′ may be printed on the solder balls 186, for example.

In FIG. 4D, at least a first upper die package 180 is placed over the bottom die 800, so that first and second edge portions 181 a, 181 b of the first upper die package 180 extend beyond the at least two edges 151 a, 151 b of the bottom die 800. In FIG. 4D, two, three or four upper die packages 180 are provided, for example in the configurations shown in FIG. 3A or 3B, as described above. The assembly is heated (as described above with reference to FIG. 2D), reflowing the solder balls to mechanically and electrically connect the front side connectors 183 of the upper die packages 180 to the contacts 110 of the package substrate 100.

The assembly shown in FIG. 4D is then encapsulated by molding compound (as shown and described with reference to FIG. 2E), and the package is optionally thinned as shown and described above with reference to FIG. 2F.

FIGS. 5A to 5D show another variation of the assembly method.

In FIG. 5A, the package substrate 100 of FIG. 1A is provided, and solder balls 186, copper posts or conductive spacers are joined to the contacts 110 of edge portions 101 a-101 d of the substrate 100. For example, solder balls 186 may be reflowed to join them to the contacts 110.

In FIG. 5B, the bottom die 800 is placed over the package substrate. The assembly is heated so the solder balls 112 reflow to join the pads of the bottom die 800 to the package substrate 100. The assembly is flushed to remove any solder paste, and the first underfill material 120 is flowed into the space between the package substrate 100 and the bottom die 800.

FIG. 5C shows the assembly after the TIM 160 is deposited on the back face of the bottom die 800. The TIM 160 may be deposited using the methods described above.

In FIG. 5D, the solder paste 170′ is applied over the solder balls 186, for example by printing. The resulting configuration is the same as shown in FIG. 4C.

Following application of solder paste, the upper die packages are placed and connected to the assembly as shown and described with reference to FIG. 4D. Then the package is encapsulated by molding compound (as shown and described with reference to FIG. 2E), and the package is optionally thinned as shown and described above with reference to FIG. 2F.

FIGS. 6A to 6F show another variation of the assembly method.

In FIG. 6A, the substrate 100 has solder balls 186 joined as shown and described above with reference to FIG. 5A.

In FIG. 6B, the bottom die 800 is placed over the package substrate 100 and the solder balls 112 are reflowed to electrically and mechanically connect the dies 154 of bottom die 800 to the package substrate 100. In this embodiment, no underfill is separately applied between the package substrate 100 and the bottom die 800 before applying the molded underfill material to the entire package.

In FIG. 6C, the TIM 160 is deposited over the back face of the bottom die 800.

The technique for depositing the TIM 160 may be the same as described above with reference to the other embodiments.

In FIG. 6D, the solder paste 170′ is printed on the solder balls 186.

In FIG. 6E, the upper die packages 180 are mounted on the TIM 160. The solder balls 186 are reflowed to electrically and mechanically connect the connectors 183 of the upper die packages 180 to the contacts 110 of edge portions 101 a-101 d of the package substrate 100. The assembly is then flushed with deionized water to remove any remaining solder paste.

In FIG. 6F, the molded underfill material 190 is inserted between the package substrate 100 and the bottom die 800, between the package substrate 100 and the upper die packages 180, and over the upper die packages, to form the package 600. Thus, a single underfill step is used, and this embodiment of the method eliminates a step of applying first underfill 120 beneath the bottom die 800 before the upper die packages 180 are connected. In some embodiments, the underfill material 190 above the upper die packages 180 is then removed, and additional thinning may optionally be performed to remove material from the back side of the upper die packages.

In some embodiments, a method comprises connecting a substrate having a plurality of integrated circuit (IC) dies to a package substrate, so that the package substrate extends beyond at least two edges of the substrate, leaving first and second edge portions of the package substrate having contacts thereon exposed. The first and second edge portions meet at a first corner of the package substrate. At least a first upper die package is placed over the substrate, so that first and second edge portions of the first upper die package extend beyond the at least two edges of the substrate. Pads on the first and second edge portions of the first upper die package are connected to the contacts of the first and second edge portions of the package substrate.

In some embodiments, an integrated circuit (IC) package comprises a substrate having a plurality of IC dies connected to a package substrate, so that first and second edge portions of the package substrate extend beyond at least two edges of the substrate. The first and second edge portions meet at a first corner of the package substrate. At least a first upper die package is positioned over the substrate, so that first and second edge portions of the first upper die package extend beyond the at least two edges of the substrate. The first upper die package has pads on the first and second edge portions thereof. The pads are connected to the contacts of the first and second edge portions of the package substrate by solder.

In some embodiments, an integrated circuit (IC) package comprises a substrate having a plurality of IC dies connected to a package substrate, so that four edge portions of the package substrate having contacts thereon extend beyond respective edges of the substrate. Each pair of adjacent edge portions meet at a respective corner of the package substrate. A plurality of upper die packages are positioned so as to partially overlie the substrate, so that first and second edge portions of each upper die package extend beyond a respective pair of adjacent edges of the substrate. The plurality of upper die packages have pads on the first and second edge portions thereof. The pads are connected to respective ones of the contacts of the package substrate by solder. A thermal interface material is provided between the substrate and the plurality of upper die packages.

In some embodiments, an integrated circuit (IC) package comprises a substrate having a plurality of bottom dies connected to the substrate, so that first and second connecting portions of the substrate extend beyond at least two edges of each bottom die, the first and second connecting portions meeting at a first corner of each bottom die. at least a first upper die package is positioned over at least one of the bottom dies, so that first and second edge portions of the first upper die package extend beyond the at least two edges of at least one of the bottom dies. The first upper die package has pads on the first and second edge portions thereof. The pads are connected to the contacts of the first and second connecting portions of the substrate by solder.

Although the subject matter has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art. 

What is claimed is:
 1. A method comprising: (a) connecting a plurality of integrated circuit (IC) dies to a package substrate with the plurality of IC dies on a first side of the package substrate, leaving at least first, second, third, and fourth edge portions of the package substrate having exposed contacts adjacent to four respective sides of one of the plurality of IC dies, the first and second edge portions of the package substrate meeting at a first corner of the package substrate adjacent to a first corner of that one of the plurality of IC dies, the respective third and fourth edge portions of the package substrate meeting at a second corner of the package substrate adjacent to a second corner of that one of the IC dies; (b) placing at least a first upper die package having a first edge portion and a second edge portion on the first side of the package substrate and over that one of the plurality of the IC dies, so as to cover and extend beyond the first corner of that one of the plurality of the IC dies in two perpendicular directions; (c) connecting pads on the first and second edge portions of the first upper die package to the contacts of the first and second edge portions of the package substrate; (d) placing a second upper die package having a third edge portion and a fourth edge portion over that one of the plurality of the IC dies that is partially overlaid by the first supper die package, so as to cover and extend beyond the second corner of that one of the plurality of the IC dies in two perpendicular directions, the second upper die package located diagonally from the first upper die package, the first and second upper die packages spaced apart so that neither of the first and second upper die packages lies over the other of the first and second upper die packages; and (e) connecting pads on the third and fourth edge portions of the second upper die package to the contacts of the third and fourth edge portions of the package substrate.
 2. The method of claim 1, further comprising: placing a third upper die package over the one of the plurality of IC dies, so that fifth and sixth edge portions of the third upper die package extend beyond the second and third edges of that one of the plurality of the IC dies, placing a fourth upper die package over that one of the plurality of the IC dies, so that the seventh and eighth edge portions of the fourth upper die package extend beyond the first and fourth edges of that one of the plurality of the IC dies, connecting pads on the fifth and sixth edge portions of the third upper die package to the contacts of the second and third edge portions of the package substrate; and connecting pads on the seventh and eighth edge portions of the fourth upper die package to the contacts of the first and fourth edge portions of the package substrate.
 3. The method of claim 1, wherein the first upper die package has solder on the pads of the first and second edge portions thereof, the method further comprising: depositing a solder paste on the contacts of the first and second edge portions of the package substrate before step (c).
 4. The method of claim 1, further comprising: placing solder on the contacts of the first and second edge portions of the package substrate; and depositing a solder paste on the solder before step (c).
 5. The method of claim 4, wherein the solder is placed on the contacts of the first and second edge portions of the package substrate after step (a).
 6. The method of claim 1, further comprising placing solder on the contacts of the first and second edge portions of the package substrate before step (a).
 7. The method of claim 4, further comprising applying a molded underfill material around the solder and above the first upper die package.
 8. The method of claim 7, wherein an underfill material is deposited between the one of the IC dies and the package substrate, between steps (a) and (b).
 9. The method of claim 7, further comprising depositing a first underfill material between the one of the IC dies and the package substrate, before applying the molded underfill material around the solder.
 10. The method of claim 1, further comprising: placing a thermal interface material over the one of the IC dies before step (b).
 11. A method comprising: (a) connecting at least one integrated circuit (IC) die to a package substrate, so that first and second edge portions of the package substrate having contacts thereon extend beyond four edges of the at least one IC die; (b) placing a first upper die package over the at least one IC die, with the at least one IC die located between the first upper die package and the package substrate, so as to cover and extend beyond a first corner of one of a plurality of the IC dies in two perpendicular directions, (c) placing a second upper die package over that IC die, so as to cover and extend beyond a second corner of that one of the plurality of the IC dies in two perpendicular directions, the first and second upper die packages positioned diagonally from each other in a plane and spaced apart so that neither of the first and second upper die packages lies over the other of the first and second upper die packages; (d) connecting pads on first and second edge portions of the first upper die package directly to the contacts of the first and second edge portions of the package substrate; and (e) connecting pads on third and fourth edge portions of the second upper die package to the contacts of the third and fourth edge portions of the package substrate.
 12. The method of claim 11, wherein step (a) includes connecting the IC die of that one of the plurality of the IC dies to the package substrate using solder, the method further comprising applying a first underfill material around the solder, between steps (a) and (b).
 13. The method of claim 12, wherein the step of applying a first underfill material includes depositing the first underfill material between the that one of the plurality of the IC dies and the package substrate.
 14. The method of claim 11, further comprising placing a thermal interface material over the that one of the plurality of the IC dies before step (b).
 15. A method comprising: (a) flip chip mounting a plurality of integrated circuit (IC) dies over a package substrate, so that first and second edge portions of the package substrate having contacts thereon extend beyond two adjacent edges of one of the plurality of IC dies; (b) placing at least a first upper die package over the one of the plurality of IC dies, so that the first upper die package is on a same side of the package substrate as the plurality of IC dies, so as to cover and extend beyond a first corner of the one of the plurality of IC dies in two perpendicular directions, (c) connecting pads on first and second edge portions of the first upper die package to the contacts of the first and second edge portions of the package substrate; (d) placing a second upper die package over the one of the plurality of IC dies that is partially overlaid by the first supper die package, so as to cover and extend beyond a second corner of the one of the plurality of IC dies in two perpendicular directions, so that the second upper die package is on the same side of the package substrate as the plurality of IC dies, the first and second upper die packages positioned diagonally from each other and spaced apart so that neither of the first and second upper die packages lies over the other of the first and second upper die packages; and (e) connecting pads on third and fourth edge portions of the second upper die package to contacts on third and fourth edge portions of the package substrate.
 16. The method of claim 15, further comprising: placing a third upper die package over the one of the IC dies, so that fifth and sixth edge portions of the third upper die package extend beyond the second and third edges of the one of the IC dies, placing a fourth upper die package over the one of the IC dies, so that the seventh and eighth edge portions of the fourth upper die package extend beyond the first and fourth edges of the one of the IC dies, connecting pads on the fifth and sixth edge portions of the third upper die package to the contacts of the second and third edge portions of the package substrate; and connecting pads on the seventh and eighth edge portions of the fourth upper die package to the contacts of the first and fourth edge portions of the package substrate.
 17. The method of claim 15, wherein step (a) includes connecting the plurality of IC dies to the package substrate using solder, the method further comprising applying a first underfill material around the solder, between steps (a) and (b), wherein the step of applying the first underfill material includes depositing the first underfill material between the one of the IC dies and the package substrate.
 18. The method of claim 15, wherein the first and second upper package dies include two different dies from the group consisting of a general purpose processor, a graphics processor, an audio/video processor, a digital radio receiver, DRAM, SRAM, or flash memory, a communications processor, a global positioning satellite (GPS) receiver, and a power management unit.
 19. The method of claim 15, wherein the first and second upper die packages are wafer level chip scale packages.
 20. The method of claim 15, further comprising thinning back surfaces of the first and second upper die packages after step (e). 